Electrical submersible pumps (ESP) are commonly used in hydrocarbon producing wells. A typical ESP includes a pump operatively coupled to a motor that is filled with a lubricant. A pressure compensator, equalizer, or seal section has a movable element that equalizes the lubricant pressure with the hydrostatic pressure of the well fluid.
The pressure compensator may have one or more bags or bellows, which are typically metal, located within a housing. Normally, the pressure compensator locates between the motor and the pump. A shaft from the motor extends through the bags or bellows. A shaft seal located at the upper end of the compensator seals against the entry of well fluid into the compensator. The typical shaft seal comprises a metal face seal that has a rotating face urged against a stationary face. Some leakage of lubricant from the compensator past the seal is desired to lubricate the faces. During filling with lubricant, the hags or bellows will be expanded when filled. A bias of the bag or bellows toward a contracted position provides a positive pressure differential of the lubricant over the hydrostatic pressure of the well fluid. The positive pressure differential assures dial lubricant may leak out, but restricts the entry of well fluid. Over time, the bias force of the bag or bellows decreases as the lubricant is depleted, lowering the positive pressure differential. Maintaining a positive pressure differential may increase the life of the ESP.